With an increased number of user equipments (UEs) such as smart phones in mobile networks, network operators are confronted with a number of new challenges. For example, users of smart phones require an experience of being always on-line, similar to the experience of being connected to the Internet via a personal computer. This always on-line experience means short reaction time for data arrival and data transmission. A natural way of solving this would be to let all smart phones in the network be always connected to the network, i.e. for a Long Term Evolution (LTE) network the smart phones would always be in the RRC_Connected state. Although, to let the smart phones be always connected consumes a lot of network resources and UE battery power.
One reason for high UE power consumption and consumption of network resources is the network controlled handover procedure for users in the RRC_Connected state. Such handover procedures cause UE random access procedure operations and RRC reconfiguration operations which increase UE power consumption and network signaling costs. This is especially true when there are many smart phones in a cell and many of them are moving with a high speed (e.g. on a train).
FIGS. 1a and 1b show simulation results for random access channel (RACH) failure and power consumption for mobile terminals travelling in a network, wherein the mobile terminals are simulated to have the same velocity at a simulation, but different velocity between each simulation. As could be seen in FIG. 1a, the number of random access failures, i.e. the number of times a random access attempt does not succeed, would largely increase when the number of smart phones in a cell increases from around 2000 smart phones/cell when the smart phones travel at 120 km/h. The random access failures are caused by many simultaneous random access attempts, at the same time as there are a limited number of preambles dedicated for handover, which leads to random access channel decoding error and collisions. As could be seen in FIG. 1b, the increase in random access failures is followed by a similar increase in UE power consumption. This is at least partly because a UE consumes lots of power when it is performing a random access procedure and because the random access failure is followed by a new random access attempt until the random access procedure is succeeded.
US2009457653 shows a procedure in which the UE is in sleep mode in a so called back-off period, which is a period between two consecutive random access procedures that may occur due to a random access failure. By setting the UE in sleep mode in the back-off periods, UE power consumption at random access procedures is decreased.
Although, there is still a need to decrease the UE power consumption at random access procedures.